2006 Philadelphia Annual Meeting (22–25 October 2006)

Paper No. 5
Presentation Time: 9:15 AM


MARENCO, Katherine N., Department of Geology, Bryn Mawr College, 101 N. Merion Avenue, Bryn Mawr, PA 19010 and BOTTJER, David, Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, kmarenco@brynmawr.edu

Bioturbation is primarily limited to bedding surfaces in Lower Cambrian rocks that were deposited in shallow subtidal siliciclastic marine shelf environments. Horizontal bioturbation, even in large quantities, commonly leads to minimal disruption of primary bedding. Thus, detection of horizontal bioturbation in vertical outcrop exposures presents a challenge. Bedding plane exposures provide the best means of obtaining information concerning the nature and quantity of horizontal bioturbation in Lower Cambrian rocks. If an accurate picture of Early Cambrian bioturbation during the initial phase of the agronomic revolution is to be obtained, precise methods for documenting and quantifying horizontal bioturbation from bedding plane surfaces are required.

Most existing methods for evaluating the nature and quantity of both vertical and horizontal bioturbation in outcrops are geared toward scoring using visual estimates of bioturbation intensity (e.g. ichnofabric indices of Droser and Bottjer (1986) and bedding plane bioturbation indices of Miller and Smail (1999)). In these methods, each numerical score corresponds to a range of bioturbation percentages. For example, bedding plane bioturbation index (BPBI) “three” corresponds to bedding surfaces that are 10-40 percent bioturbated. Thus, describing a bedding plane as “BPBI three” can be misleading, particularly if the amount of bioturbation present falls close to either of the two extremes.

A simple new method, presented here, employs image analysis to overcome the limitations of the bedding plane bioturbation index method for precisely evaluating quantities of horizontal bioturbation on bedding surfaces. Field photographs of 600cm2 areas of individual bedding planes were each overlain by a grid. Each square within the grid was marked according to whether bioturbation was present or absent in that square. The total number of bioturbated vs. non-bioturbated squares was then used to calculate the percentage area of the bedding surface that was bioturbated. As demonstrated with new data from the Lower Cambrian of eastern California, this method can be used as a lab-based complement to the field-based bedding plane bioturbation index method or as a stand-alone means of arriving at accurate percentages of bioturbation on studied bedding planes.